This invention relates to methods and apparatus for optical/digital inspection and to the suppression and enhancement of optical features. In one exemplary aspect, the present invention relates to the real-time suppression of periodic features and the enhancement of non-periodic defects, for example in the photomasks used in integrated circuit (IC) fabrication.
The present invention is described below with emphasis on the inspection of periodic grids or masks such as the photomasks used to pattern the etch and doping steps used in integrated circuit fabrication. In a particularly useful application, the technique is used to suppress the periodicity of IC masks and, at the same time, enhance or maximize the image intensity of non-periodic defects. However, as will be evident to those of skill in the art, the combination of Fourier optics and linear systems technology with nonlinear optics technology which is used for this defect enhancement is applicable in general to enhance and/or suppress the intensity of optical features.
The scale of integration of semiconductor devices on integrated circuit chips has improved greatly in the last several years. In fact, over the past five years, the silicon IC technology has grown from large scale integration (LSI) to very large scale integration (VLSI), and is expected to grow to ultra-large scale integration (ULSI) during the next several years. This continued improvement in silicon integrated circuit integration has been made possible in part by the apparatus used for lithography and etching. Generally, the density of integrated circuits and their speed of operation are dependent upon the accuracy and resolution of the lithography and etching apparatus which is used to form patterns of circuit elements in masking layers on the semiconductor wafer and then precisely replicate those patterns from the masking layers in the underlying semiconductor wafer layer(s). As the minimum lithographic feature size is decreased towards one micrometer and below, and as the device density increases accordingly, integrated circuits become increasingly susceptible to defects, including those resulting from the starting material and from the integrated processing sequence itself, and those which are transferred or replicated from the mask into the underlying semiconductor wafer layers. In fact, defect densities as low as one per square centimeter can result in unacceptably low integrated circuit processing yields.
In the past, digital techniques have been used for the inspection of two-dimensional fields such as integrated circuit masks. Generally, such techniques use a dual scanning microscope system and sophisticated algorithms for comparison and detection. Typically, however, such digital techniques are complicated and time consuming.
Unlike digital techniques, optical systems offer the advantage of parallel processing. Furthermore, there is no excessive requirement of accuracy in the output in terms of the actual intensity at each point. Rather, it is sufficient that the signal associated with a defect be much stronger than the signal associated with the surrounding periodic or quasi-periodic structure, so that, for example, a thresholding operation can be used to determine the defect location.
Optical spatial filtering techniques to perform defect enhancement have been examined in the past with regard to such applications as the inspection of electron beam collimating grids and silicon diode array targets for television camera tubes, as well as to the inspection of photomasks used in integrated circuit manufacture. The optical spatial filtering systems used a filter in the Fourier plane to attenuate the discrete spatial frequencies of the periodic portion of the mask, so that, upon retransformation only defects were present in the output. While the results of such systems were promising, the usefulness of the technique was limited by the fabrication time of the filter, and by the need to use high quality, low f number lenses when inspecting objects of large dimensions.
Recently, this second constraint has been removed by employing holographic recording of the output combined with phase-conjugate read-out. See, for example, R. L. Fusek, et al, "Holographic Optical Processing for SubMicron Defect Detection", Proceedings of the SPIE, Vol. 523, January, 1985. A three-step process is involved, however: first, a photographic filter is made, recording the Fourier spectrum of a mask to be tested; second, a hologram of the mask is recorded in the output plane using the filter to block the periodic portion of the spectrum; and finally, after processing the hologram is illuminated by the phase-conjugate of the reference beam and the defect-enhanced image is found in the output plane. This method has been used to detect submicron defects. However, it has the disadvantage, in addition to the above-mentioned requirement of a three-step process, that the process must be tailored to each mask and type of mask. That is, a new hologram must be recorded for each mask to be inspected and a new filter must be made for each type of mask.